As a glass for a glass fiber excellent in strength and elastic modulus, a glass (S glass) composed of a glass composition of SiO2, Al2O3 and MgO is conventionally known. However, with respect to the S glass, the production of the glass fiber is not always easy from the viewpoint of the 1,000-poise temperature and the liquid phase temperature. Here, the 1,000-poise temperature refers to a temperature at which the melt viscosity of a glass becomes 1,000 poises, and the liquid phase temperature refers to a temperature at which the crystal deposition takes place for the first time when the temperature of a molten glass is decreased. Generally since glass fibers can efficiently be produced in the case where glass is spun by making the melt viscosity of the glass nearly 1,000 poises, spinning is usually carried out in the temperature range (working temperature range) between the 1,000-poise temperature and the liquid phase temperature. With respect to the S glass, the working temperature range is narrow, and the molten glass is liable to crystallize (devitrify) even under an influence of only a slight temperature decrease. Therefore, in order to carry out stable spinning, in the production step of glass fibers, the spinning condition needs to be precisely controlled.
As an improved S glass, glass compositions containing SiO2, Al2O3, MgO and CaO are known (Patent Literature 1 and Patent Literature 2 shown below). Patent Literature 1 discloses a glass composition easily fiberized along with a decrease in the liquid phase temperature. Patent Literature 2 discloses a glass composition in which a difference is large between a temperature (fiberization temperature) corresponding to the viscosity of a near 1,000-poise temperature and a maximum temperature (liquidus) at which an equilibrium is present between a liquid glass and a primary crystal phase thereof.